The problem starts when the true victims - often the abuser's "nearest and dearest" - adopt his/her point of view and begin to feel guilty and responsible for his/her reprehensible behaviors. This folie a deux (literally, in French, "madness in twosome") or shared psychosis is very common: victims and abusers form symbiotic dyads, abrogate reality, and share the same delusions. They allocate roles: the victim triggers the abuse and deserves it, the abuser is merely a hapless tool, devoid of volition and with an absent impulse-control.
But why would anyone succumb to such a patently fallacious view of the world? Why would anyone assume the guilt for her own torture and maltreatment? Shared psychosis is a complex phenomenon with numerous psychodynamic roots. Some victims fear abandonment and would do anything to placate their abusive intimate partner. Others grew up in dysfunctional families and are familiar and comfortable with abuse (it is their "comfort zone".) Some victims are masochistic and others simply want to "make the relationship work." Fear plays a big part, too: sometimes the only way not to provoke another onslaught is by playing by the abuser's rules.
So, what can you do about it?
(From the book "Malignant Self-love: Narcissism Revisited" by Sam Vaknin - Click on this link to purchase the print book, or 16 e-books, or 3 DVDs with 16 hours of video lectures on narcissists, psychopaths, and abuse in relationships: http://www.narcissistic-abuse.com/thebook.html)